1. Field of the Invention
The present invention relates to a method of manufacturing an optical module having a semiconductor light-emitting or -detecting device of a metal-cap package structure and a lens which are held in alignment with each other by a housing of synthetic resin. More particularly, the present invention relates to a method of manufacturing such an optical module by assembling the semiconductor light-emitting or -detecting device and the housing through two fixing stages, i.e., a provisional fixing stage based on the high-frequency induction heating of a first thermosetting resin and a final fixing stage based on the atmospheric heating of a second thermosetting resin.
2. Description of the Related Art
Optical modules are an optical assembly comprising a semiconductor device, e.g., a semiconductor light-emitting device such as a laser diode or a semiconductor light-detecting device such as a photodiode, and a lens that are held in alignment with each other. Such optical modules are used in the field of optical communications, etc. For example, computers for data communications have a pair of optical modules, i.e., a semiconductor light-emitting device and a semiconductor light-detecting device, mounted on a board. Specifically, those optical modules comprise a semiconductor device, a lens, and a housing which holds the semiconductor device and the lens and retains the ferrule of an optical plug which is connected to the optical module. When the optical plug is connected to the optical module, the semiconductor device and the optical fiber in the ferrule are optically coupled to each other by the lens.
The lens incorporated in the optical module is mostly a spherical lens because it can easily be manufactured to high dimensional precision only by a mechanical process, is inexpensive, and can easily be assembled as it has no directivity and hence requires no orientational adjustments upon installation. Other lenses that find use in optical modules include an aspherical lens and a gradient-index rod lens. These lenses are normally fixedly assembled in respective housings by a bonding or fusing process. According to the bonding process, the lens is dropped into and positioned in a lens mount cavity in the housing, and thereafter coated with an adhesive of synthetic resin, which is then thermoset to secure the lens in place. According to the fusing process, after the lens is placed in the lens mount, a glass ring having a low melting point is placed around the lens and then melted with heat to hold the lens in position.
Heretofore, the housing which holds the semiconductor device and the lens has been a metal housing in most applications. Recently, however, housings molded of synthetic resin are widely used because they can easily be manufactured, can hold lenses securely in place, and can be manufactured inexpensively. For example, these molded housings can incorporate a lens that is assembled as a built-in lens according to an insert molding process, or can support a lens with a plurality of teeth. The semiconductor device is mostly in the form of a semiconductor element sealed by a package which comprises a metal cap.
Generally, a semiconductor device of a metal-cap package structure is fixedly mounted in a housing of synthetic resin with a built-in lens by coating a thermosetting resin such as an epoxy resin on an outer circumferential surface of the stem of the package along an end of the housing, and then thermosetting the resin in a heated atmosphere or at room temperature.
According to the above assembling process, the lens and the semiconductor device need to be maintained in their respective positions for accurate optical axis alignment by a jig while the thermosetting resin is being coated on the stem and until the coated thermosetting resin is fully thermoset. Since it takes several tens of minutes before the coated thermosetting resin is fully thermoset, the assembling procedure has been highly inefficient. If the housing is made of metal, then the semiconductor device can be fixed to the housing quickly, efficiently, and firmly by a welding process. However, such a welding process is not applicable to the molded housing.
One possible solution would be to bond the semiconductor device quickly to the housing with an ultraviolet-curable adhesive. However, the ultraviolet-curable adhesive is disadvantageous in that it fails to provide a sufficiently high level of bonding strength, tends to leave an uncured layer due to adhesive flows on an inner housing wall, and has a relatively low level of weather resistance.